drm/exynos: fix linux framebuffer address setting.

With iommu, buffer->dma_addr has device addres so this patch
fixes for physical address to be set to fix.smem_start always.

Signed-off-by: Inki Dae <inki.dae@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>